We have seen that Marx in 1845, and Engels in 1888 and 1892, introduce the criterion of practice into the foundation of the theory of knowledge of materialism{48}. Apart from practice, to pose the question of whether "objective (i.e., objective) truth corresponds to human thinking," is scholasticism, – Marx says in the 2nd thesis on Feuerbach. The best refutation of Kantian and Humian agnosticism, as well as other philosophical vagaries (Schrullen), is practice, – Engels repeats. "The success of our actions proves the correspondence (Übereinstimmung) of our perceptions with objective (objective) nature of the things we perceive," – Engels objects to the agnostics{49}.

Compare this with Mach's reasoning about the criterion of practice. "In everyday thinking and ordinary speech, one usually contrasts the apparent, the illusory, with reality. Holding a pencil before us in the air, we see it in a straight position; lowering it at an angle into water, we see it bent. In the latter case one says: 'the pencil appears bent, but in reality it is straight.' But on what ground do we call one fact reality and reduce the other to the status of illusion?.. When we make the natural mistake of expecting the occurrence of usual phenomena even in unusual cases, our expectations are certainly deceived. But the facts are not to blame for this. To speak of illusion in such cases makes sense from a practical point of view, but not at all from a scientific one. In the same way, the frequently discussed question of whether the world really exists, or is merely our illusion, no more than a dream, makes absolutely no sense from a scientific point of view. But even the most absurd dream is a fact, no worse than any other" ("Analysis of Sensations," pp. 18–19).

It is true that not only an absurd dream, but also absurd philosophy, is a fact. One cannot doubt this after becoming acquainted with Ernst Mach's philosophy. Like the most vulgar sophist, he confuses the scientific-historical and psychological investigation of human errors, all manner of "absurd dreams" of humanity such as belief in forest spirits, household spirits, and the like, with the epistemological distinction between the true and the "absurd." This is the same as if an economist said that both Senior's theory, according to which the whole profit of the capitalist comes from the "last hour" of the worker's labor, and Marx's theory, are equally facts, and from a scientific point of view it makes no sense to ask which theory expresses objective truth and which expresses bourgeois prejudices and the venality of its professors. Joseph Dietzgen saw in scientific, that is, materialist, theory of knowledge a "universal weapon against religious faith" ("Kleinere philosophische Schriften," p. 55[78]), but for the ordinary professor Ernst Mach, "from a scientific point of view, it makes no sense" to distinguish between materialist theory of knowledge and subjective-idealist theory! Science is impartial in the struggle between materialism and idealism and religion – this is a favorite idea not only of Mach, but of all modern bourgeois professors, these, by the just expression of the same Joseph Dietzgen, "credentialed lackeys, stupefying the people with labored idealism" (p. 53, ibid.).

This is precisely such labored professorial idealism, when the criterion of practice, which distinguishes illusion from reality for everyone and each person, is placed by E. Mach outside the bounds of science, outside the bounds of the theory of knowledge. Human practice proves the correctness of the materialist theory of knowledge, – said Marx and Engels, declaring "scholasticism" and "philosophical vagaries" attempts to solve the fundamental epistemological question apart from practice. For Mach, practice is one thing, and the theory of knowledge is quite another; they can be placed side by side without one conditioning the other. "Knowledge," says Mach in his latest work: "Knowledge and Error" (p. 115 of the second German edition), "is a biologically useful (förderndes) mental experience." "Only success can separate knowledge from error" (116). "A concept is a physical working hypothesis" (143). Our Russian Machists, wishing to be Marxists, with astonishing naiveté accept such phrases of Mach as proof that he is drawing closer to Marxism. But Mach here draws as close to Marxism as Bismarck drew close to the workers' movement, or Bishop Evlogii to democracy. With Mach, such propositions stand side by side with his idealist theory of knowledge, and do not determine the choice of one or another definite line in epistemology. Knowledge can be biologically useful, useful in human practice, in the preservation of life, in the preservation of the species, only if it reflects objective truth, independent of man. For the materialist, the "success" of human practice proves the correspondence of our representations with the objective nature of things which we perceive. For the solipsist, "success" is all that I need in practice, which can be considered separately from the theory of knowledge. If we include the criterion of practice in the foundation of the theory of knowledge, we inevitably obtain materialism, – says the Marxist. Let practice be materialist, but theory is a separate matter, – says Mach.

"Practically," he writes in the "Analysis of Sensations," "carrying out any actions whatsoever, we can as little dispense with the representation of the I, as we cannot dispense with the representation of the body when reaching our hand for something. Physiologically we remain egoists and materialists with the same constancy with which we continually see the sun rising. But theoretically we ought not to adhere to this view at all" (284–285).

Egoism here is beside the point, for it is a category not at all epistemological. The apparent motion of the sun around the earth is equally beside the point, for one must include in practice, which serves us as the criterion in the theory of knowledge, also the practice of astronomical observations, discoveries, and so forth. What remains is Mach's valuable admission that in their practice people are guided entirely and exclusively by the materialist theory of knowledge, while the attempt to bypass it "theoretically" expresses only the pedantic-scholastic and labored-idealist strivings of Mach.

How little new these efforts are to separate out practice as something not subject to examination in epistemology, in order to clear space for agnosticism and idealism, is shown by the following example from the history of German classical philosophy. Standing on the path from Kant to Fichte is G. E. Schulze (so-called in the history of philosophy Schulze-Aenesidemus). He openly defends the skeptical line in philosophy, calling himself a follower of Hume (and among the ancients – Pyrrho and Sextus). He decisively rejects any thing-in-itself and the possibility of objective knowledge, decisively demands that we not go beyond "experience," beyond sensations, anticipating and answering an objection from another quarter: "Since the skeptic, when he participates in life's affairs, recognizes the reality of objective objects as indubitable, conducts himself accordingly and admits a criterion of truth, – then the skeptic's own conduct is the best and most evident refutation of his skepticism"[79]. "Such arguments," Schulze answers indignantly, "are suited only for the rabble (Pöbel, p. 254), for my skepticism does not affect life's practice, remaining within the bounds of philosophy" (255).

Likewise, the subjective idealist Fichte also hopes within the bounds of idealist philosophy to find a place for that "realism, which is unavoidable (sich aufdringt) for all of us and even for the most resolute idealist, when it comes to action, realism accepting that objects exist completely independently of us, outside us" (Werke, I, 455).

"So it comes down to food and drink in discussing the question of the ideality or reality of the world? – exclaims the indignant idealist. – How base! What a violation of good manners to curse materialism in the scientific sense with all one's might from the philosophy chair and from the theology chair, in order to practice materialism in the crudest sense at table!" (195). And Feuerbach exclaims that equating subjective sensation with the objective world "means equating a nocturnal emission to childbirth" (198).

A remark not of very good manners, but it hits the mark precisely for those philosophers who teach that sense perception is the reality existing outside us.

The point of view of life, of practice, must be the first and fundamental point of view of the theory of knowledge. And it inevitably leads to materialism, throwing out at the threshold the infinite fabrications of professorial scholasticism. Of course, in this one must not forget that the criterion of practice can never, from the very nature of the case, fully confirm or refute any human representation whatsoever. This criterion is also so "indefinite" as not to allow human knowledge to transform itself into an "absolute," and at the same time so definite as to wage a relentless struggle against all varieties of idealism and agnosticism. If that which our practice confirms is the only, the final, objective truth, – then from this follows recognition of the only path to this truth as the path of science standing on the materialist point of view. For example, Bogdanov agrees to recognize objective truth for Marx's theory of money circulation only "for our time," calling it "dogmatism" to ascribe to this theory "transhistorical-objective" truth ("Empiriomonism," book III, p. VII). This is again a confusion. The correspondence of this theory to practice cannot be changed by any future circumstances for the same simple reason for which it is eternal truth that Napoleon died on May 5th, 1821. But since the criterion of practice – that is, the development of all capitalist countries over recent decades – proves only the objective truth of Marx's entire socio-economic theory in general, and not of this or that part, formulation, and so forth, it is clear that to talk here of "dogmatism" in Marxists means making an inexcusable concession to bourgeois economics. The only conclusion from that opinion, shared by Marxists, that Marx's theory is objective truth, consists in the following: proceeding along the path of Marx's theory, we shall be approaching objective truth more and more (never exhausting it); proceeding, however, along any other path, we can arrive at nothing but confusion and falsehood.